Feature: M2 parsing improvements

[fallenoak]

This PR is aimed at cleaning up M2 parsing in preparation for several improvements to M2 rendering in Wowser.

Merging will break Wowser until https://github.com/wowserhq/wowser/pull/143 is merged.

Changes

• Added parsing of second UV coordinate pairs in vertex struct
• Added parsing of bone lookups
• Added parsing of blending overrides
• Renamed several attributes and structs for clarity/consistency

[timkurvers]

Awesome! 👍